Because of a continued trend to reduce the size of electronic circuits, thick film circuits are being used more frequently in many commercial products. Ceramic thick film circuit substrates often take the place of traditional printed wiring boards. The thick film substrates are smaller and also more refined and dimensionally more stable than the traditional printed wiring boards. Consequently, semiconductor chips may be mounted directly onto the substrates rather than through more space consuming "Dual Inline Packages" (DIPs).
Thick film circuits also have been chosen as alternates to thin film circuits. However, while thick film circuits may be manufactured at a lower cost than thin film circuits, the thick film circuits are typically also less precise than their thin film counterparts. The greater precision of thin film circuits results from the photolithographic processes by which thin film patterns are generated.
Thick film circuits, on the other hand, are generated by repeatedly screening pastes through patterned masks onto a substrate. After each screening operation the screened-on paste pattern is fired to harden it and to thereby transform it into a permanent feature of the substrate. Each level of such screened and fired patterns typically differs from its adjacent levels in the physical layout of the pattern and in the electrical characteristics of the applied paste. Various pastes produce different patterned features on the substrate, which include conductors, resistors of various resistivities, and insulators.
Masks for the various pattern levels have to be aligned with respect to each other to produce the desired circuits. Misalignment between different mask levels may occur because of nonuniform wear of reference guides, which is typically caused by extreme abrasive characteristics of the typical ceramic material of the substrates. Also, after an initial alignment of the masks, it is possible for the masks to creep out of an initial optimum alignment position because of stresses to which the masks are subjected during repeated screen printed operations. In contrast to the screen printing masks, photolithography masks used in thin film production processes are not subject to such stresses.
According to known thick film forming techniques, a first level mask generates a plurality of sets of alignment fiducial marks. Each subsequent level mask includes in its printing pattern a uniquely positioned feature which matches and corresponds in its optimum alignment position to a corresponding fiducial mark printed on the substrate through the first level mask. The masks of such subsequent levels become aligned with respect to each other through the fiducial marks printed through the first level mask.
Inaccuracies in the boundaries of the screened ink patterns in each of two patterns which are referenced against each other in each alignment tend to decrease the precision of the thick film circuits. In addition, there exists an inherent problem of aligning the first level mask to the nominal edge of the substrates. It has been found that a majority of the substrates have edges which deviate from an ideal true edge position. However, it is particularly desirable to adjust the overall printed pattern to the mean or true edge of all the substrates. Such an adjustment limits a maximum error displacement of the pattern from an edge to an error no greater than half the distance between two worst case error displacements in opposite directions.